There have been known methods to indirectly detect whether a tire has a decreased air pressure by making a relative comparison among angular velocities of tires attached to a vehicle (see Patent Literature 1 for example).
Furthermore, as a relative comparison method, there has been known a method of using three types of determination values DEL1, DEL2, and DEL3 (see Patent Literature 2 for example). In this method, DEL1 is a wheel speed comparison value obtained from comparison between the wheel speeds of wheels on one diagonal line and the wheel speeds of wheels on the other diagonal line, DEL2 is a wheel speed comparison value obtained from comparison between the wheel speeds of front wheels and the wheel speeds of rear wheels, and DEL3 is a wheel speed comparison value obtained from comparison between the wheel speeds of right-side wheels and the wheel speeds of left-side wheels.
Among the three types of determination values, DEL2 is a wheel speed comparison value obtained from comparison between the wheel speeds of front wheels and the wheel speeds of rear wheels. When two wheel tires of the front axis or the rear axis of a vehicle have a simultaneous deflation, DEL2 changes significantly. Thus, the use of DEL2 described above provides in principle the sensing of two wheels of the same axis having a decreased pressure in the vehicle.